There's a great free ebook Eloquent JavaScript that introduces the language and some more advanced concepts in the further chapters.
Depending on your background, there might also be more specific resources like Javascript for Java Developers. I read a little book with the same title once (it's only available in German, afaik) and it helped me a lot getting started and reusing my existing Java/programming knowledge from university.
So maybe you provide some more details about your background and then we can help you further. :)